Fossil fuels (or petroleum-based fuels) have formed the basis for energy production and transportation in the 20th and 21st centuries. Increased need among growing populations and emerging nations, as well as market volatilities arising from wars, politics, and natural disasters, have focused world-wide attention on this non-renewable resource. In particular, rising costs and threats of shortages and supply interruptions have recently highlighted the need for alternative fuel sources to petroleum-based products. Biofuels have particularly been a focus for alternative fuels.
Biofuel is generally regarded as being any fuel derived from biomass. The term biomass is often used in regard to plant-based sources, such as corn, soy beans, flaxseed, rapeseed, sugar cane, and palm oil, but the term can generally extend to any recently living organisms, or their metabolic byproducts, that play a part in the carbon cycle.
The production of biofuels to replace fossil fuels is in active development, focusing on the use of cheap organic matter (usually cellulose, agricultural waste, and sewage waste) in the efficient production of liquid and gas biofuels which yield high net energy gain. Biofuels are viewed as environmentally favorable (particularly over fossil fuels) because the carbon in biofuels was recently extracted from atmospheric carbon dioxide by growing plants, and burning the biofuels does not result in a net increase of carbon dioxide in the Earth's atmosphere. Perhaps more importantly, biofuels are a renewable fuel source, and the potentially limitless fuel supply derived therefrom can have a stabilizing effect on fuel prices in the long-term.
One widespread use of biofuels is in home cooking and heating (e.g., wood, charcoal, and dried dung). Biologically produced alcohols, most commonly methanol and ethanol, and to a lesser extent propanol and butanol, can be produced through enzymatic and microbiological fermentation. For example, ethanol produced from sugar cane is widely used as automotive fuel in Brazil, and ethanol produced from corn is being used as a fuel additive in the United States. Gases and oils are also being produced from various waste sources. For example, thermal depolymerization of waste materials (including plants, food, paper, plastic, paint, cotton, synthetic fabrics, sewage sludge, animal parts, and bacteria) allows for extraction of methane and other compounds similar to that obtainable from petroleum.
The need for alternative fuel sources, and particularly biofuels, also extends to high end uses, such as automobile and jet fuels. Almost all high end use fuels (such as jet engine fuel, diesel engine fuel, and gasoline engine fuel) are presently made from petroleum. Accordingly, such fuels are prepared through refining of crude oils. Refining generally encompasses three basic categories of activities: separation, upgrading, and conversion. During separation, feedstock (e.g., crude oil) is separated into two or more components based on some physical property, typically boiling point. The most common separation method is distillation. Upgrading uses chemical reactions to improve product quality by removing unwanted compounds that impart undesirable properties. For example, “sweetening” relates to removal of mercaptans and other organosulfur compounds, which are corrosive. Hydroprocessing uses hydrogen and a catalyst to remove reactive compounds, such as olefins, sulfur compounds, and nitrogen compounds. Clay treating removes polar compounds by passing the fuel stream over a bed of clay particles. Conversion fundamentally changes the molecular structure of the feedstock, usually by cracking large molecules into small molecules (e.g., catalytic cracking and hydrocracking).
FIG. 1 provides a schematic layout of a modern, fully integrated refinery for preparing various fuel types. As seen in FIG. 1, crude oil is fed to the distillation column where straight-run light and heavy gasoline, kerosene, and diesel are separated at atmospheric pressure. The bottoms from the atmospheric column are vacuum distilled to obtain gas oils for fluid catalytic cracking (FCC) or hydrocracker feed. Previously, the vacuum residue might have been used as a low-value, high-sulfur fuel oil for onshore power generation or marine fuel. To remain competitive today, however, refiners must collect as much high-value product as possible from every barrel of crude, and vacuum residue may now be sent to a residue conversion unit, such as a residue cracker, solvent extraction unit, or coker. These units produce additional transportation fuels or gas oils, leaving an irreducible minimum of residue or coke.
The jet fuel produced by a refinery may be all straight-run or hydroprocessed product, or it may be a blend of straight-run, hydroprocessed, and/or hydrocracked product. Small amounts of heavy gasoline components also may be added. Straight-run kerosene from low-sulfur crude oil may meet all of the jet fuel specification properties. Straight-run kerosene, though, is normally upgraded by mercaptan oxidation, clay treating, or hydrotreating before it can be sold as jet fuel. The refinery must blend the available streams to meet all performance, regulatory, economic, and inventory requirements. Sophisticated computer programs have been developed to optimize all aspects of refinery operation, including the final blending step. The refiner really has only limited control over the detailed composition of the final jet fuel product. It is determined primarily by the composition of the crude oil feed, which is usually selected based on considerations of availability and cost. Moreover, the chemical reactions that occur in the conversion process are not specific enough to allow for much tailoring of the products.
The consumption of transportation fuels continues to grow worldwide, particularly in light of the rapidly growing need for transportation in emerging economies. For example, just the consumption of jet fuel in the United States increased from 32 million gallons per day in 1974 to 70 million gallons per day in 1999. Although fuel needs are obviously growing, the number of refineries has not kept up with the growing need. According to the National Petrochemicals and Refiners Association, the last refinery built in the United States was completed in 1976. Between 1999 and 2002, refining capacity in the United States rose only 3 percent. Moreover, public perception and environmental concerns make building new refineries more and more difficult. For example, a report from the California Energy Commission notes that even though 10 refineries representing 20% of the state's refining capacity were closed between 1985 and 1995, it is unlikely that new refineries will be built in California. Accordingly, not only is there a need for increased amounts of transportation fuels, there is also a need for alternative sources (to combat dwindling petroleum supplies and the vicissitudes of the crude oil market) and alternative methods of preparing fuels.
In its Broad Agency Announcement (BAA) 06-43 posted Jul. 5, 2006, the Defense Advanced Research Projects Agency (DARPA) Advanced Technology Office (ATO) began soliciting proposals for biofuels to explore energy alternatives and fuel efficiency efforts to reduce reliance on oil to power its aircraft, ground vehicles, and non-nuclear ships. DARPA's BAA06-43 particularly sought efforts to develop a process that efficiently produces a surrogate for petroleum based military jet fuel (such as the current standard fuel, JP-8) from oil-rich crops produced by either agriculture or aquaculture (including but not limited to plants, algae, fungi, and bacteria) and which ultimately can be an affordable alternative to petroleum-derived JP-8.
Biodiesel has been proposed as an alternative source for jet fuel production; however, current biodiesel alternative fuels are produced by transesterification of triglycerides extracted from agricultural crop oils. Specifically, fats are reacted with alcohols and converted to alkyl esters (biodiesel) followed by conversion of biodiesel to jet fuel. The overall reaction is provided below in Formula (1),CH2(OCOR1)CH(OCOR2)CH2(OCOR3)+3ROH+(catalyst)→R1OCOR+R2OCOR+R3OCOR+CH2OHCHOHCH2OH  (1)wherein R1, R2, and R3 represent possibly distinct hydrocarbon chains. As seen in Formula (1), one molecule of triglyceride is combined with three alcohol molecules to produce three molecules of biodiesel and one molecule of glycerol. Thus, the transesterification reaction converts the triglyceride triester to three fatty acid alkyl monoesters. This process unacceptably yields a blend of methyl esters (biodiesel) that is 25% lower in energy density than JP-8 and exhibits unacceptable cold-flow features at the lower extreme of the required JP-8 operating regime (−47° C.). For example, kinetic viscosity at 40° C. of fuel prepared in this manner is in the range of 1.9 to 6.0 centistokes, but the viscosity of an acceptable jet fuel should be in the range of about 1.2 centistokes. Further, it is common for such fuels to have a freezing point in the range of about 0° C. Moreover, as feedstock cost is the primary production cost driver in the preparation of jet fuel from biomass, there has heretofore been no process for preparing jet fuel from biomass that is affordable and utilizes a suitably available necessary feedstock material.
There is likewise an increasing need and desire to establish viable alternative fuel sources for other transportation vehicles, particularly automobiles. Alternative fuels, as defined by the Energy Policy Act of 1992 (EPAct), include ethanol, natural gas, propane, hydrogen, biodiesel, electricity, methanol, and p-series fuels. As previously pointed out, biofuels represent a potentially limitless fuel supply that has heretofore been virtually inaccessible. The ability to use biomass as a source for automobile fuels, such as gasoline or diesel, could not only potentially provide lowered gasoline prices due to increased supply but also lessen the demand for crude oil and stem the fear of waning reserves.
Vegetable oils, animal fats, and algae lipids can be converted to a combination of liquid and gaseous hydrocarbons by transesterification, deoxygenation, pyrolysis, and catalytic cracking processes. All of these processes have been developed to varying degrees over the past 100 years. To convert these fuelstocks into fuel, some combination of these processes can be employed, and optimal combination is a function of both the fuelstock and the desired properties of the fuel product. The present invention provides a process for preparing fuel from biomass.